Research On Dynamic Characteristics Of Vehicle ESP Hydraulic System

Vehicle ESP hydraulic system is composed of several components, which work cooperatively driven by the Electronic Control Unit of ESP. According to different driving cycle, the hydraulic system exerts corresponding braking pressure to realize active intervention. As the actuator of ESP, the hydraulic system is a high-speed response system, every control valve of which responds in a short time. So, the dynamic characteristics of the hydraulic system influence the control quality and reliability of ESP remarkably. Based on the work of the control logic of ESP, the dynamic characteristics of the hydraulic system are studied in the paper.First, the structure and operating principle of one hydraulic system are analyzed. Through the analysis, AMESim models, including hydraulic unit, master cylinder with precharge unit and braking wheel cylinder, are identified. Meanwhile, based on the theories of hydromechanics, mathematic models reflecting the main components such as restrictor, control valve, accumulator and pump are described, which provide theoretical basis for simulation and design.Based on the theories of dynamic characteristics, the design indexes of system are suggested. Through the simulation, several results are got, including the varieties of pressure, flow rate and volume of wheel cylinder. So are the variety of characteristic parameter of accumulator, pump and interrelated parameter of control valves. These results reflect dynamic characteristics of the system well. Simulation are also carried out about six influencing factors including the structure of control valve, pump, accumulator, precharge unit, fluid and wheel cylinder. The results indicate that every factor has important affection on the characteristics and the factors couple in each other. So, it is important to take account of matching during the process of design.According to the theories of Vehicle Dynamics, the nonlinear tire model is adopted, and the four-wheel vehicle model with seven degrees of freedom and the vehicle reference model are set up on this basis. The control logic based on the feedback of yaw rate is presented and the scheme of braking outside front wheel and inner rear wheel is established. The hydraulic system and dynamics model are linked through the interface technology between AMESim and Simulink and co-simulations are carried out for matching analysis. Different characteristic parameters are analyzed and a group of parameters are drawn up for certain vehicle, which are validated in different driving cycle. The simulation results indicate that the parameters of hydraulic system match the entire vehicle well and can eliminate the errors between the actual yaw rate and the reference effectively to enhance the lateral stability of vehicle.